1. Field of the Invention
This invention relates to a reflection type liquid crystal display device. More particularly, it relates to a reflection type liquid crystal display device of low power consumption employing a sole polarizer plate and which is capable of achieving high picture quality and high response characteristics.
2. Description of the Related Art
A liquid crystal display device (LCD) is lightweight, thin in thickness and low in power consumption, and is finding use in a wide field of application ranging from a small size device for computers or timepieces to a large size device such as a word processor or a personal computer.
From now on, use of the LCD as a display device for a portable information terminal or a personal data assist (PCD) is thought to be promising. In order for the LCD to be used as a display device for PCD, it is required to reduce the size and the weight of the device further and to reduce power consumption such as to enable driving for a prolonged time by e.g., an electric cell. As a display device capable of coping with these demands, a reflection type LCD is stirring up notice.
Among the reflective type LCDs, there is such a device employing two polarizer plates and a twist-nematic liquid crystal (TN liquid crystal) or a super-twist liquid crystal (STN liquid crystal). This reflection type LCD has a liquid crystal layer of the TN or STN liquid crystal sandwiched between two polarizer plates and a reflection plate on one side of the sandwiched structure.
Among the reflective type LCDs, there is also such a device employing a guest-host (GH) system. In this GH system, a two-color dyestuff (guest) is dissolved in a liquid crystal (host) and liquid crystal molecules are controlled in orientation by an electrical field to change the direction of orientation of dyestuff molecules simultaneously to utilize the change in light absorption due to dichroism in order to make display. This system, not in need of polarizer plates, may be exemplified by a phase change-GH system exploiting the phase transition, referred to hereinafter as a PC-GH system, a chiral nematic-GH system which has overcome the problem of hysteresis by exploiting the merit proper to the GH system, referred to hereinafter as the CN-GH system, an a-N*-GH system which randomly orients the liquid crystal to eliminate hysteresis to enable gradation representation, a .lambda./4-GH system employing a quarter wave plate, and a three-layer GH system having a three-layer liquid crystal layer structure of the GH system.
Of the above described reflection type reflection type liquid crystal display devices, that employing two polarizer plates reportedly is insufficient in lightness and difficult to design as a color type device because the incident light has to traverse the polarizer plates four times before exiting the device so that 70% of the incident light is absorbed by the polarizer plates without being used for display.
Conversely, the above-mentioned reflection type liquid crystal display device employing a sole polarizer plate can be improved in lightness by diminishing absorption by the polarizer plate.
With the above-mentioned reflection type liquid crystal display device of the GH system not employing the polarizer plate, lightness can similarly be assured.
The following Table 1 summarizes the properties of the reflection type liquid crystal display device employing the sole polarizer plate and the reflection type liquid crystal display device of the GH system not employing the polarizer plate.
TABLE 1 angle re- of light- con- grada- sponse visibi- driving mode ness trast tion speed lity voltage sole STN .DELTA. .largecircle. .circle-w/dot. .DELTA. .DELTA. up to polar- liquid 7V izer crystal plate TN .DELTA. .circle-w/dot. .circle-w/dot. .circle-w/dot. .DELTA. up to liquid 3V crystal OCB .DELTA. .circle-w/dot. .circle-w/dot. .circle-w/dot. .largecircle. up to liquid 3V crystal no PC-GH .largecircle. .largecircle. .DELTA. .largecircle. .largecircle. up to polar- 5V izer CN-GH .largecircle. .largecircle. .circle-w/dot. .largecircle. .largecircle. up to plate 5V a-N* .largecircle. .largecircle. .circle-w/dot. .largecircle. .largecircle. up to GH 5V .lambda./4 GH .largecircle. .largecircle. .circle-w/dot. .circle-w/dot. .largecircle. up to 5V three- .circle-w/dot. .largecircle. .circle-w/dot. .DELTA. .DELTA. up to layer 5V GH times 3
In Table 1, lightness, contrast, gradation, response speed and the angle of visibility are evaluated in three stages. Specifically, .circle-w/dot., .largecircle. and .DELTA. denote extremely satisfactory, satisfactory and acceptable, respectively.
As may be seen from Table 1, the reflection type liquid crystal display device of the GH system is extremely satisfactory in lightness, herein white color display. However, the GH system cannot be said to be sufficient in reliability of the component material.
It cannot be said that all characteristics can be met sufficiently with the remaining types of the reflection type liquid crystal display device.
Recently, with progress in the polarizer plates, a material superior to the conventional material in light transmittance and polarization has been developed. Thus, attention is directed to the reflection type liquid crystal display device employing a sole polarizer plate. Moreover, since the use of the STN liquid crystal enables simple matrix driving, attention is directed to a reflection type liquid crystal display device employing a sole polarizer plate and the STN liquid crystal. However, this reflection type liquid crystal display device achieves high contrast by twisting the array of the liquid crystal molecules by 180.degree. to 270.degree. such that it is not optimum in picture quality or response speed.